1. Field of the Invention
The present invention generally relates to a gate drive and, more specifically, to a gate drive for an insulated gate bipolar transistor (IGBT).
2. Description of Related Art
Power electronic based power supplies are becoming more common in commercial industry and military applications. Such power supplies are replacing their mechanical counterparts (i.e., motor generator set), because they offer higher reliability, better regulation and smaller size than the mechanical counterparts. The power electronic transistor that is widely used in these power supplies is an insulated gate bipolar transistor (IGBT).
With reference to FIG. 1, IGBT, denoted generally as reference numeral 1, has three terminals: collector 3, gate 5, and emitter 7. Typically, an IGBT gate drive connects to gate terminal 5 and emitter terminal 7; collector terminal 3 and emitter terminal 7 are connected to the power portion of the circuit. As the voltage from gate 5 to emitter 7 increases above a threshold voltage (about 5 volts), IGBT 1 turns on and current can flow from collector 3 to emitter 7. IGBT 1 is typically operated in switch mode (i.e., either fully on or fully off). This switch mode minimizes the power dissipation in the device.
With reference to FIG. 2 and with continuing reference to FIG. 1, IGBTs are commonly designed to only conduct current in one direction and typically a free wheeling diode (FWD) 9 is placed inside IGBT 1 module, which is electrically connected anti-parallel to IGBT 1. FWD 9 provides a path for an inductive load current to flow when the IGBTs are off.
An IGBT is typically connected at its emitter to one end of a load whose other end is connected to a power supply voltage. The power supply voltage may range from a few volts to several thousand volts, and the load current through the load and the IGBT may range from the milliampere region to a hundred or more amperes. In order to minimize the power dissipation across the IGBT, a large turn-on voltage (e.g., 12-15 volts) is normally applied to the gate of the IGBT to cause the IGBT to be operated in the saturation region with its collector-to-emitter voltage (Vce) in the order of a few volts. However, a problem occurs if, when the IGBT is fully turned on and carrying a large current, the load is shorted. The IGBT is then subjected to an excessive power dissipation condition due to the high current through the IGBT and the rising voltage developed across the collector-to-emitter of the IGBT. If a short circuit condition exists (or develops) and persists, the IGBT will fail due to the excessive power dissipation. This factor and others conspire to increase the chances of latch-up if the short-circuit condition persists for too long before the device is turned off.
It is, therefore, necessary to turn off an IGBT if, and when, a fault condition, such as a short circuit, develops or exists. Prior art schemes for turning off an IGBT subjected to a short circuit condition rely on applying a relatively sharp turn-off voltage to the gate of the IGBT when a short circuit condition is sensed. However, it has been discovered that trying to turn off an IGBT sharply and rapidly when the IGBT is carrying a very large current and is subjected to a short circuit condition may cause the IGBT to lose control and fail. Thus, if the IGBT loses control over the load current flowing through it while its Vce keeps increasing, the IGBT will fail due to excessive power dissipation. Another scheme for turning off an IGBT subjected to a short circuit condition maintains the full turn-on voltage applied to the gate for a time interval of several microseconds after the detection of a short circuit condition in order to maintain control. During this time interval, the Vce of the IGBT rises towards the load supply voltage. Following the time interval, the turn-on voltage is removed. However, while the IGBT is on, its power dissipation is very high, and the IGBT may fail due to excessive power dissipation.
The problems present in the prior art are significantly reduced in IGBT gate drives in accordance with the present invention.